Array API Reference
This function is to handle array variables. About Array variables, please take a look at Script and variables.
The Array Variable is one whose name starts with "@".
This function is frequently used with for loop. And when you use SELECT SQL and check the size of the record, this function is used.
This "Array.size()" function returns size of Array. This function will be the most used one.
This example is the case it is used with SELECT SQL sentence.
If the argument is null, this function returns 0.
Therefor when you write a function to return Array Variable, you can return null if there is no array's value to return.